Having 1 card necessary for your win hurts a lil bit. Have you thought about running Monastery Swiftspear? She seems to do well with many of the strengths of this deck, particularly as it seems you usually are lacking a turn 1 play(apart from a tapland).

I'd assume it's for budget reasons, but just in case you didn't know; fetchlands like Flooded Strand and Wooded Foothills are generally better than Terramorphic Expanse and Frontier Bivouac

I would also note that running Treasure Cruise seems a bit too dangerous in this deck, with your dependence on a well stocked graveyard. Divination or something like it seems by far preferable in this deck.

Finally, while Satyr Wayfinder brings forth the danger of milling a chimera, he is 1 mana cheaper than Scout the Borders for only 1 less card milled. That speed may be critical enough to merit the risk.

Didn't think of Monastery Swiftspear, thanks! Will try out soon, could replace Hornet Nest as an alternative win condition.
I believe Treasure Cruise is good, as losing 2-3 power to chimera doesn't hurt that much, provides 1 more card than Divination, and can cost as low as 1.
Satyr Wayfinder isn't instant/sorcery and can't fetch a chimera/hornet nest. That hurts a lot in a deck reliant on those two creatures.

Hornet Queen. If you're gunna run the nest, might as well go all out of rthe big guns

Hornet Queen is three green mandatory. If he wants to keep his mana where it is his chance of being able to cast it would be very low.

Commune with the gods is worth a second look if you'd fix your mana a bit more. Dumping up to 4 extra power into your creature would work in your favor with even faster grab for it. An extra mana saved could go into a t4+ combo if you got the draw.

I'm amazed you don't have Become immense among your consideration at all since I'd throw enough dead cards in that it'd be viable to delve out and swing with a chimera that much bigger.

Satyr wayfinder gets a body on your board and adds delve fuel, not to be dismissed so fast.

A 2-of of Restock/Pull from the Deep/Reviving Melody could be useful.

With all the Hornet Nests and the potential tokens they can generate, why not a 1 of Obelisk of Urd, naming chimeras? It'd help your Spellheart Chimera to swing earlier, and easily puts them out of range of most burn removal. Then you can save your counters for straight removal instead.

ch1m3ra: A turn 2 Commune with the Gods doesn't do any good, because playing Spellheart Chimera turn 3 is a risky play (with no mana to protect it). Scout the Borders adds stability to the deck.
Become Immense would be good with a higher number of creatures. In this deck, it would often be a dead card.
Archimedes75: Not sure if you realize this, but Hornet Nest tokens aren't chimeras.

Omedog: Yes, I do realize this. But Obelisk of Urd has convoke, which means you can use the tokens to help you cast it. Since you don't have much mana ramp, that'll help you get it out quicker.

Most of the time, when a chimera is onboard, it dispatches our opponent in 2-3 turn. So +3 power is not that valuable. +3 toughness for 6mana 1card... not that efficient.
We could name insect and attack with 3/3 insects, but then it's just a win faster card -- we could do the same with 1/1 insects. The only time it matters is when our opponent has mass removal.
All this is assuming we have those insect tokens. If we don't, Obelisk of Urd just doesn't do anything.